The present invention relates to analog-to-digital converters (ADCs), in particular to reference voltage error correction therein.
ADCs convert an input analog signal to a digital representation (e.g., digital word). Typically, ADCs use a reference voltage in their conversion process. For example, in successive approximation register (SAR) ADCs, reference voltages are used in bit trials to calculate bit values of the digital word. However, reference voltage sources often include non-idealities that can corrupt the conversion.
Pure analog approaches are conventionally used to generate reference voltages that are ideally process, supply voltage and temperature (PVT) independent. However, three main issues arise from pure analog approaches. First, high power consumption and complexity associated with these approaches negate most of the benefits provided from them. Second, accurate PVT tracking may be difficult to impossible as reference requirements are getting more stringent in today's applications. Third, regardless of precision of the reference voltage generation, non-idealities are prone to enter the system. For example, reference voltage often depends on the input signal in many ADC designs, which introduce reference voltage errors. Theoretically, it may be possible to eliminate Vin dependency if sufficient settling time between cycles is allowed; however, allowing this much time can cause significant delays and other errors for high speed, high accuracy ADCs.
One time calibration, such as calibration after manufacturing time or powering up, can alleviate some errors introduced by process variation but cannot compensate for other errors, including dynamic cycle-to-cycle errors. A separate error correction path, such as a feedback loop, using a separate highly accurate ADC to measure errors can be used to correct slow errors, for example supply and/or temperature induced errors. However, separate ADC error correction paths must use expensive ADCs to measure these slow errors, and still they are not fast enough to correct faster errors such as reference voltage errors due to input signal dependency, because these errors change cycle to cycle.
Therefore, the inventors recognized a need in the art for fast reference voltage error corrections in ADCs without an extra ADC that can dynamically correct such errors cycle to cycle.